A number of machines have been proposed for producing bottles from plastic resin materials. One of the methods presently widely used to produce containers, such as bottles, from a thermoplastic resin, such as polyethylene, is called "blow molding."
In the blow molding process the plastic resin, generally in the form of pellets, is placed in a hopper. The hopper leads to an extrusion screw which, by means of pressure or heat and pressure, causes the solid pellets to flow into a viscous semi-fluid mass. The semi-fluid resin is forced by pressure to flow around a mandrel and through an extrusion die orifice which shapes it into a thin-walled elongated tube called a "parison."
At this stage in the blow molding process the parison, having just been forced from the extruder, is hot; for example, it may be about 350.degree. F. The parison is then positioned over a blow tube and within an opened two-part mold. The mold is then closed and its sealing members seal the top and the bottom of the parison to form an air-tight unit. Air is then blown through the blow tube and into the inside of the parison, causing it to expand against the inner walls of the mold. The air is under high pressure, for example, 80 pounds per square inch, and is injected into the parison in a quick burst of pressure. The walls of the mold are cooled by circulating cold water, or a mixture of water and anti-freeze, in a constant stream through an outer jacket surrounding the mold face. The cold walls of the mold cool the resin and form the parison into a bottle. The air is then exhausted and the two mold halves are opened.
The mold is constructed in two halves which must be brought together quickly and in exact alignment in order to prevent escape of air. The top of the mold is sometimes called a "neck ring." The mold will form the body of the bottle and "flashing," usually at the top and bottom of the bottle. Since the opposite walls of the parison are hot and collapse together under pressure, they may coalesce to form the sheet-like flashing. The flashing consists of the collapsed opposite walls of the parison tube which have been brought together and which must subsequently be trimmed off from the bottle.
After the bottle is formed, the mold halves are opened and the bottom flashing may be used to pull the bottle out of the mold. This action of pulling the bottle will also remove the formed bottle from around the blow pipe. The bottles may be removed by fingers, i.e., metal snap members, which reach up and grab the bottom flashing, sometimes called a "tail." The fingers pull the bottle down onto a conveyor belt which conveys the bottles from the blow molding machine to subsequent machines in which the flashing is removed. The flashing material, which is generally at the neck of the bottle and at its bottom, may be re-used to form additional bottles.
This type of blow molding machine is generally described in U.S. Pat. No. 3,369,272; and U.S. Pat. No. 3,470,582 describes a somewhat similar machine. Both of these patents are assigned on their faces to Hoover Ball and Bearing Company. In U.S. Pat. No. 3,369,272 an "annular shearing edge 70" of "portions 68 of the mold sections 60" is used for sealing and shearing the neck of the formed plastic resin bottle. In U.S. Pat. No. 3,470,582 the "mold cavity neck portion 68" is used for separation of the parison.
From time to time there have been certain problems with this type of blow molding machine if a "pre-finish" neck is utilized, i.e., a neck formed by a shearing action in the mold. Upon closure of the mold parts, a sealing and shearing member is moved into a cylindrical hollow on the blow pin, leaving a gap filled by the resin parison, to form a seal using the parison. Then the sealing neck member attached to the blow pipe, called a "blow pin," is moved axially upwards; and the rising blow pin shears the plastic resin of the neck by forcing it against the shearing and sealing member which remains level.
One problem may arise because the blow pipe is not supported at its bottom end. The blow pin is supported only by the blow pipe, which is a thin and lengthy tube and which must carry the compressed air and so is necessarily hollow. The blow pipe is under considerable thermal stress as it is surrounded by a fast-moving hot tube of formed plastic resin and intermittently receives blasts of compressed air. Due to all these factors, and due to human error and inexperience in both the axial and lateral adjustment of the blow pin, it may happen that the blow pin is not exactly centered or it is too high or too low. Not being centered means that an imaginary axis drawn through the center of the hollow and cylindrical blow pin would not be aligned with, or coincide with, the imaginary central axes of the two halves of the mold.
The non-centering or non-exact alignment of the blow pin may cause difficulties because the mold is closed under great pressure. If the blow pin has been misaligned, the sealing and shearing member, sometimes called "shear steels" or "shear rings," which meet the blow pin, may strike it with great force. This may damage the blow pin or, more frequently, may damage or cause undue wear or stress on those members.
These problems may be either aggravated or lessened by the presence of the plastic resin parison. The plastic resin parison acts as a lubricant and prevents wear which might otherwise occur between the metal members. On the other hand, the parison may present considerable difficulty in the event of non-exact alignment between the shearing member and the blow pin. If there is a misalignment of mis-matching, the plastic may become too thin, causing holes. If there is a hole, it will permit escape of the compressed air upon the compressed air stroke, resulting in a deformed or nonformed bottle.
One feature of the present invention provides a plastic resin severing and mold closure device to be utilized in a two-part mold blow molding machine. In such a blow molding machine a plastic resin parison is extruded in tubular form around a blow tube, and the blow tube delivers compressed air to expand the parison to form against the internal walls of a two-part separable mold having a top neck portion. The blow tube carries a cylindrical blow pin having a cylindrical shoulder portion and a cylindrical shear portion. The severing and mold closure device is removably affixed at the top neck portion of each of the two mold parts.
The closure device comprises a base member, preferably of an aluminum alloy, in the approximate form of a one-half ring seen in top view; and the base member has a flat top portion and a descending flange portion. The flange portion forms an inverted shelf seen in side view, and the base member is of a heat conductive metal alloy. The closure device further comprises a top insert, which is a seal insert in the form of a generally flat member and is approximately a one-half ring. The seal insert has an inner edge with an inner diameter which is close to, but not touching, the shoulder of the blow pin in order to compress, but not shear, the parison and thereby permit blow pressure with the parison upon closure of the mold.
A bottom insert, which is a shear insert, is in the form of a generally flat member and is approximately a one-half ring member. The shear insert has an inner edge which meets in a shearing action against the shear portion of the blow pin to shear the parison.
Both the seal insert and the shear insert are made of heat tempered, i.e., heat hardened, steel. The inner edge of the shear insert has an inner diameter which is less than the inner diameter of the inner edge of the seal insert. The closure device also provides means to removably fasten the shear insert to the seal insert and to the base member.